High strength, corrosionresistant alloy



United States Patent 3,082,082 HIGH STRENGTH, .CORROSIQN- RESESIANTE ALLQY Lawg ence R. Bitlwell', Dayton, and Franklin H. Beck and Mars GJFontana; Columbus, 'Qhio, assignors to The Ohio State University Research Foundation, Columbus, Ohio, acorporation of Ohio Dt ti gv Fitss fieut- .18. 1. .5. 5. .NQ- ,696

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The present invention relates to ferrous base, corrosionresistant alloys, and' rnore" specifically to an improved corrosion-resistant"alloy of the chromiummickel-iron type which may be readily cast and which is characterized by relatively "high mechanical strength properties and its ability to be hardened without loss of 'its corrosionresistant 12 r-operti'es.

This application is a continuation-impart of our prior, copen-ding application Serial No. 598,746, filed July 19, 1956, now abandoned, entitled High Strength, Corrosion- Resistant Alloy.

One of the primary objectionable characteristics of socalled stainless or corrosion-resistant steels, typified by the 18 Cr and 8 Ni steels, is their inability to be heattreated to provide desirable high mechanical strength properties of hardness, high tensile and yield strength, and good ductility without impairment to their corrosionresistant properties. As is well known to those skilled in the art, attemptsto harden 18 and 8 stainless steel through precipitation hardening methods result in a mixture of structural phases within the alloy which greatly reduces its normal corrosion resistance.

While we are aware that certain alloys, containing a relatively higher content of chromium and a relatively owe wres of ni kel saturated w th 1am s in- 1888 steel, have been d sclo ed. such rior Thi h chr m min n k a l y have gqntained a a e eme t the-reef tani m out were 992 od h wrou ht a her han a P u t W here found. throu h expe n ha 1W? estur n e .rslatirq y h h Perce f am o latively low pe ce 9 2 e t e h r wi m bd num QQPPQL when. QSlllQQI! and mang s havingeven a relatively small percentage of titanium present therein are relatively brittle and display poor corrosion resistance. Further, we have noted that certain prior art alloys containing relatively high percentages of chromium, relatively low percentages of nickel, together with molybdenum popper, but which do not include carbon and silicon, display poor hardness and aging charaoteristics, and are inoap able of producing commercially ce abl st n s Accordingly, the present invention has for its primary objective the provision of a chi omium-nickel-iron alloy which is, characterized by ability to produce excellent castings, which has good corrosion resistance and which, at the same time, is capable of being heat-treated to obtain excellent mechanical strength properties Without impairment of its corrosion-resistant properties.

It is another object of this invention to provide a corrosion-resistant alloy from which cast articles may be readily formed and heat-treated to obtain high mechanical strength, non-seizing, and non-gelling properties.

A further object of the present invention is to provide a cast ferrous base alloy having corrosion-resistant properties superior to those of 18 and 8 stainless steel and which, at the same time, may be heattreated to possess a hardness of from 70 to 100% greater than 18 and 8 stainless steel, an increaseof 100% or more in yield strength and an increase in tensile strength up to 50% over cast 18 and 8 stainless steel.

In accordance with the present invention, we have found that excellent high strength, heat-hardena'ble, corrosionresistant alloys may be formed from a solid solution composed essentially of 24 to 28% chromium, 3 to 6.5% nickel, 0.01 to 0.08% carbon, 1.5 to 2.5% molybdenum, 2.5 to 3.5% copper, 0.3 to 2% silicon, up to 2% manganese, and the balance iron. We have found through numerous actual tests and experiments that the usual impurities found in stainless steels, such as minor percentages of sulfur, phosphorous, etc., will not alter the desired properties of the present'alloy, but proportions of the individual elements out of the foregoing ranges, or the addition of relatively minor proportions of other metals, such as titanium, columbium, and tantalum, results in more brittle alioys with inferior corrosion resistance, and inferior mechanical properties.

To attain maximum mechanical strength and corrosionresistant properties, the following composition range is preferable Percentage by weight The present alloy may be readily cast and thereafter hardened either by heating and solution quenching or by age hardening procedures. Where it is desired to age harden the present alloy, the carbon content thereof is preferably kept below 0.03%, as we have found that the corrosion resistance of the alloy upon age hardening is impaired when the carbon content is in excess of 0.03%.

As a specific example of the practice of the present invention, an alloy havi-ng the desired corrosion-resistant and high mechanical strength properties was prepared by combining in accondance'with standard alloying procedures, 24.80% chromium, 5.42% nickel, 0.026%carbon, 0.60% manganese, 1.87% molybdenum, 3.22% copper, 0.89% silicon, and the remainder iron with the usual impurities present in standard or commercial purity alloying elements and compositions. The chromium, molybdenum, silicon, manganese, and carbon were added to the melt in ferro-alloy forms, while the nickel, copper, and the major proportion of iron were added in substantially pure forms. The molten alloy was then poured into molds to form several standard, cast, tensile strength test specimens. One such specimen was heated to 2050 F for 2 hours and then .water quenched.

This latter specimen when tested revealed a Brinel-l hardness of 267, a yield strength (0.2% oifset) of 88,000 p.s.i., a tensile strength of 113,000 p.s.i., and 19% elongation in 2 inches.

Another test specimen formed from the foregoing melt was tested following age hardening by maintaining the specimen at a temperature of 900 F. for 1 hour and thereafter permitting the same to cool in atmospheric air to room temperature. This age hardened specimen re. vealed a Brinell hardness of 336, a yield strength (0.2% offset) of 128,750 p.s.i., a tensile strength of 143,500 p.s.i. with 5.2% elongation in 2 inches.

aosaoaa Specimens of the foregoing melt were tested for corrosion resistance by immersion in a 50% sulfuric acid solution at room temperature for 48 hour periods and showed no corrosion. Such specimens were also tested in boiling 65% nitric acid solution for 48 hour periods and revealed an average corrosion rate of only mils per year.

Specimens of the foregoing melt which had been age hardened at 900 F. for 4 hours and then air cooled were tested for corrosion in boiling 65% nitric acid for 48 hour periods and revealed an average corrosion rate of only 9 mils per year.

As another example, an alloy having the desired corrosion-resistant and high mechanical strength properties was prepared by combining 25.50% chromium, 5.45% nickel, 0.05% carbon, 0.48% manganese, 1.87% molybdenum, 2.84% copper, 0.87% silicon, and the remainder iron. The resultant alloy was then poured into molds to form several standard, cast, test specimens. One specimen from this melt was heated to 2050 F. for 2 hours and then water quenched. This specimen, when tested for hardness, revealed a Brinell hardness of 260, a yield strength (0.2% offset) of 87,000 p.s.i., a tensile strength of 114,000 p.s.i., and elongation in 2 inches. Additional specimens from this melt were tested for corrosion resistance by immersion in a 50% sulfuric acid solution at room temperature for 48 hour periods and showed no corrosion. Such specimens were also tested in boiling 65% nitric acid solution for 48 hour periods and revealed an average corrosion rate of only 10 /2 mils per year.

A third melt comprising 27.66% chromium, 5.30% nickel, 0.057% carbon, 0.46% manganese, 2.06% molybdenum, 2.66% copper, 0.89% silicon, and the remainder iron was prepared in accordance with the foregoing procedures, and specimens thereof were tested for mechanical strength and corrosion resistance with the fol- *lowing results: Brinell hardness of 280, yield strength (0.2% otfset) of 82,000 p.s.i., tensile strength of 117,000 p.s.i., and 11% elongation in 2 inches; no corrosion resulting from immersion in a 50% sulfuric acid solution at room temperature for 48 hours; an average corrosion rate of only 7 /2 mils per year when immersed in a boiling 65 nitric acid solution for 48 hour periods.

The present alloy, both solution quenched and age hardened, is further characterized by its non-galling and non-seizing properties, thus rendering the same highly useful in the formation of high strength, screw-threaded articles.

In view of the foregoing, it will be seen that the present invention provides an improved, readily castable, high strength, corrosion-resistant alloy which is characterized by its ability to be heat-treated and hardened to provide mechanical strength properties greatly in excess of those attainable with the so-oalled 18 and 8 stainless steels. At the same time, the present alloy is formed from readily available and comparatively inexpensive, non-precious metals of standard or commercial purity, and may be produced on a comparative economic basis with ordinary types of stainless steels.

We claim:

1. A corrosion-resistant alloy characterized by its ability to produce usable cast products and to be heattreated to provide high mechanical strength properties 4 without loss of corrosion resistance, said alloy being composed essentially by weight of Percent by weight Chromium 24-28 Nickel 3-6.5

Carbon 0.01-0.08

Molybdenum 1.5-2.5 Copper 2.5-3.5 Silicon 0.3-2

Manganese 0-2 Iron 68.69-55.42

2. A corrosion-resistant alloy consisting apart from impurities of the following:

Percent by weight Chromium 24.00-28.00

Nickel 3.00-6.50

Carbon 0.01-0.08 Molybdenum 1.50-2.50 Copper 2.50-3.50 Silicon 0.30-2.00 Manganese 0.00-2.00 Iron Remainder and being characterized by its ability to produce commercially acceptable castings and to be heat-hardened without impairment of its corrosion resistance.

3. A corrosion-resistant alloy consisting apart from impurities of about 24 to 27.5% chromium, about 4.75 to 6% nickel, about 0.01 to 0.03% carbon, about 1.75 to 2.25% molybdenum, 2.75 to 3.25% copper, about 0.3 to 1% silicon, up to about 0.75% manganese, and the balance iron, said alloy being characterized by its ability to produce commercially acceptable castings and to be age hardened without impairment to its corrosion-resistant properties.

4. A corrosion-resistant alloy characterized by its ability to produce commercially acceptable castings and to be heat-treated to provide high mechanical strength properties without loss of corrosion resistance, said alloy consisting apart from impurities of approximately:

Percent by weight Chromium 24.8,0 Nickel 5.42

Carbon 0.02

Manganese 0.60 Molybdenum 1.87 Coppen 3.22 Silicon 0.89

Iron 63.18 

1. A CORROSION-RESISTANT ALLOY CHARACTERIZED BY ITS ABILITY TO PRODUCE USABLE CAST PRODUCTS AND TO BE HEATTREATED TO PROVIDE HIGH MECHANICAL STRENGTH PROPERTIES WITHOUT LOSS OF CORROSION RESISTANCE, SAID ALLOY BEING COMPOSED ESSENTIALLY BY WEIGHT OF: PERCENT BY WEIGHT CHROMIUM - 24-28 NICKEL - 3-6.5 CARBON - 0.01-0.08 MOLYBDENUM - 1.5-2.5 COPPER - 2.5-3.5 SILICON - 0.3-2 MANGANESE - 0-2 IRON - 68.69-55.42 